1 . Field of the Invention
The present invention relates to a method of efficiently producing a plurality of diamond crystals, and more specifically, it relates to a method of synthesizing diamond crystals by growing diamond on diamond seed crystals.
2. Description of the Prior Art
Diamond is a material having the highest hardness and modulus of elasticity among the existing materials. Further, extremely pure diamond has such superior characteristics that it has the highest thermal conductivity and high transmittance of light in infrared regions. Thus, diamond is one of the precious resources that cannot be replaced by other materials.
There are generally two types of methods for synthesizing diamond. In one of such methods, carbon material is mixed or brought into contact with a solvent metal such as iron, cobalt or nickel to be subjected to diamond-stable superhigh pressure and temperature whereby the carbon is converted into diamond under the action of the solvent metal. According to such method, the solvent metal penetrates into the carbon material (generally prepared by graphite), whereby the carbon is caused to diffuse through the solvent metal which is in the form of a thin film, to generate diamond. The driving force for diamond generation in this method is the difference in solubility between graphite and diamond with respect to the solvent metal under a constant temperature. According to this method, diamond is spontaneously nucleated, and rapidly grows until it reaches a certain size. A considerable quantity of fine diamond powder has generally been synthesized by this method to be applied to, e.g., abrasives.
However, larger diamond crystals of high quality cannot be synthesized by the aforementioned method.
On the other hand, a method of synthesizing large diamond crystals of good quality is disclosed in U.S. patent No. 3,297,407 issued on Jan. 10, 1967 to R. H. Wentorf, Jr. FIG. 1 roughly illustrates a synthesizing vessel as employed in this method. Such prior art method is now described with reference to FIG. 1.
Diamond seed crystals 1 are located in upper and lower portions of the synthesizing vessel as shown in FIG. 1. A carbon source 2 is provided between the two diamond seed crystals 1, with solvent metal layers 3 arranged in the upper and lower sides of the carbon source 2. A cylindrical heater 4 is adapted to heat the synthesizing vessel.
The synthesizing vessel in the above structure is heated to the maximum temperature at its central portion in the axial, i.e., vertical direction, while the upper and lower end portions thereof are heated at relatively low temperatures.
Carbon dissolved in the solvent metal layers 3 under diamond-stable superhigh pressure and temperature is precipitated as diamond on the seed crystals 1, which are under relatively low temperatures. This method utilizes the difference in solubility of carbon with respect to the solvent metal based on temperature difference, and is called a temperature difference method. Such temperature difference method enables growth of diamond only from diamond seed crystals, while enabling control of diamond growth rate by maintaining the temperature difference at a prescribed value. Therefore, a large diamond of one carat size can be laboratorially synthesized according to said method.
However, in the aforementioned temperature difference method, a superhigh pressure generator required for the synthesis process is extremely expensive and a considerable time is required for the synthesis process. Therefore, the cost for synthesizing diamond is greatly increased, and hence no large diamond has been produced in the industrial field in practice.
The technique of synthesizing diamond based on the aforementioned temperature difference method is further described in "Some Studies of Diamond Growth Rates" by R. H. Wentorf, Jr., the Journal of Physical Chemistry, Vol. 75, No. 12, 1971, pp. 1833-1837. FIG. 2 shows a synthesizing vessel employed in the prior art as disclosed in this literature. The difference between the synthesizing vessels as shown in FIGS. 1 and 2 resides in that a partition wall is provided at the central portion in the axial, i.e., vertical direction of the vessel as shown in FIG. 2, and both vessels are substantially identical to each other in other points. As also shown in FIG. 2, the synthesizing vessel is provided with vertically symmetrical temperature distribution such that the vessel is under a relatively high temperature at its axial center, with temperatures being lowered toward the upper and lower ends thereof.
According to an experiment made in practice in the synthesizing vessel as shown in FIG. 2, a large diamond crystal of good quality can be synthesized in the lower part of the vessel, whereas no good diamond crystal can be grown in the upper part thereof even if the temperature distribution and materials of components are prepared to be the same as those of the lower part. The reason for this is described in the aforementioned literature by Wentorf, Jr. (see pages 1834 to 1835), and is considered to be based on the action of gravity. In other words, when the solvent metal is molten and carbon is dissolved in the solvent metal, the solvent metal is reduced in specific gravity. Such reduction in specific gravity follows increase in temperature. Under such circumstances, dissolution of the carbon in the solvent metal is caused in the upper end portion of the solvent metal in the lower part of the synthesizing vessel, while the temperature is raised higher in the said upper end. However, the result is contrary to this in the upper part of the synthesizing vessel. Thus, the specific gravity of the solvent metal is at the minimum in the lower end of the upper vessel part, whereby convection is caused by the action of gravity. Consequently, the carbon is excessively supplied to the upper end on which the seed crystal is located, leading to impossibility in retaining the appropriate rate for growing good diamond crystals. Thus, according to the conventional method of synthesizing diamond based on the temperature difference, good diamond crystals can be synthesized only in the lower part of the synthesizing vessel as shown in FIG. 2. Therefore, the cost for synthesizing diamond is further increased in the conventional method utilizing the temperature difference method.
Another example of a method of synthesizing diamond on the basis of temperature difference is disclosed in Japanese patent laying open gazette No. 88289/1977, which points out other disadvantages of the conventional method of synthesizing diamond by the temperature difference method. The disadvantages pointed out in the prior art literature are that, in the conventional process of synthesizing diamond based on the temperature difference method, spontaneous nucleation takes place in portions other than the seed crystals thereby to reduce the yield of synthesized diamond crystals as grown on the seed crystals, that it is difficult to obtain diamond crystals of uniform quality and size, and that grown diamond crystals tend to be cracked. The Japanese patent laying open gazette No. 88289/1977 then discloses a method of preventing generation of diamond crystals in portions other than the seed crystals. This method comprises a step of placing a shielding layer for preventing nucleation between the solvent metal layer and the seed crystals, which shielding layer is prepared by a transition metal such as cobalt (Co), iron (Fe) and manganese (Mn) applicable as a solvent to diamond or a carbide-generating element such as titanium (Ti), chromium (Cr) and tungsten (W).
However, according to experiments made by the inventors, it has been found that the method disclosed in Japanese patent laying open gazette No. 88289/1977 still has the following disadvantages: Although the transition metal such as Co, Fe and Mn can effectively be employed as the material for the shielding layer for preventing nucleation when the solvent metal is prepared by a metal having low carbon solubility such as nickel (Ni) or Fe-Ni alloy, such shielding layer of the transition metal cannot effectively prevent nucleation when the solvent metal is prepared by that having high carbon solubility such as Fe, Co or an alloy mainly composed of these metals. Further, employment of the element which generates a carbide, such as W and Cr, leads to deterioration in quality of finally obtained diamond crystals, which is not preferable since fine crystals of carbides such as W and Cr generated simultaneously with the diamond crystals tend to be trapped in the diamond crystals.